The present disclosure relates generally to the field of logging tools and particularly to electromagnetic logging tools. More specifically, the present disclosure relates to various directional antenna and tool designs, and methods for directional electromagnetic well logging while drilling.
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques use instruments or tools equipped with transmitters adapted to emit energy into a subsurface formation that has been penetrated by a borehole. In this description, “instrument” and “tool” will be used interchangeably to indicate, for example, an electromagnetic instrument (or tool), a wire-line tool (or instrument), or a logging-while-drilling tool (or instrument). The emitted energy interacts with the surrounding formation to produce signals that are then detected and measured by one or more sensors. By processing the detected signal data, a profile of formation properties can be generated.
More specifically, electromagnetic logging tools including electromagnetic induction and wave propagation logging tools are used for determination of electrical properties of formations surrounding a borehole. Such logging tools obtain measurements relating to resistivity (or its inverse, conductivity) of the formation that, when interpreted, illustrate various petrophysical properties of the formation and fluids therein. The physical principles of electromagnetic induction resistivity well logging are well known.
Electromagnetic logging tools use transmitter and receiver antennas formed from an antenna coil, or a coil for short, formed of one or more turns of conductor wire, wound around a support. In some embodiments, such antennas may be operable as transmitter and/or receiver. Those skilled in the art will appreciate that an antenna may be used as a transmitter at one time and as a receiver at another. It will also be appreciated that the transmitter-receiver configurations disclosed herein are interchangeable due to the principle of reciprocity, i.e., the “transmitter” may be used as a “receiver”, and vice-versa.
Conventional electromagnetic logging tools employ non-directional transmitter and receiver antennas that do not have azimuthal sensitivity. In high angle or horizontal wells, measurements obtained with non-directional, antennas do not contain information about the directionality of the formation that allows distinguishing whether the borehole is approaching, e.g., a conductive layer from above or below. Such information is used in well placement applications. The logging tools comprising a directional antenna (an antenna having azimuthal sensitivity) such as those described in U.S. Pat. No. 5,508,616, U.S. Pat. No. 6,163,155, U.S. Pat. No. 6,476,609. Such logging tools can provide a directional measurement containing information about the directionality of the formation. It further provides more information used for various formation evaluation applications.
One challenge of logging-while-drilling electromagnetic logging tools is to protect the antenna coil in order to withstand the demanding harsh downhole conditions. One design to protect the antenna known in prior art is to provide a portion of the collar as “necked-down”, by radially reducing the collar dimension to provide a deep and wide groove. The antenna coil is wound about the reduced radial dimension area of the collar, separated by an insulating layer from the surface of the reduced area, and a slotted sleeve or shield is provided and secured in position over the antenna coil. Various antenna designs of this type are described in U.S. Pat. No. 4,536,714, U.S. Pat. No. 4,949,045, U.S. Pat. No. 6,297,639, and U.S. Pat. No. 6,566,881.
Another design known in the prior art is to provide the antenna coil directly in a pathway within a drill collar having grooves through the thickness of drill collar, without the slotted shield disposed about the antenna coil found in the first method described. The antenna coil in the pathway exposed in the grooves is protected by a non-metallic material disposed in the grooves, and the antenna coil in the pathway between grooves is protected by the material of the drill collar through which the pathway passes. Various antenna designs of this type are described in U.S. Pat. No. 5,138,263, U.S. Pat. No. 5,530,358, U.S. Pat. No. 6,577,129, U.S. Pat. No. 7,057,392, U.S. Pat. No. 7,414,407, and U.S. Pat. No. 7,839,149.
Various processing methods for generating measurements from data collected by logging-while-drilling electromagnetic logging tools including a directional antenna are described in U.S. Pat. No. 7,382,135 and U.S. Pat. No. 8,195,400.